The invention relates to moisture curable compositions.
Pressure sensitive adhesives are useful for a variety of purposes and in various applications. These adhesives may be applied to a first substrate, and may then be utilized to form a bond to a secondary substrate or surface at a later date with the application of light pressure. Pressure sensitivity requires a certain balance of elastic and viscous properties. The glass transition temperature and modulus (G') at application temperature have been found to be important properties for pressure sensitive adhesive performance as stated in the Handbook of Pressure Sensitive Adhesive Technology, Satas, Second Edition, 1989, p. 172. Pressure sensitive adhesive often have a room temperature storage modulus of no more than about 2.times.10.sup.6 dynes/cm.sup.2 at 75.degree. C. and a Tg of about -30.degree. C. to about 10.degree. C. depending on whether the composition is a permanent grade, removable grade or freezer grade adhesive. Typically pressure sensitive tape adhesives have high storage moduli. Pressure sensitive adhesives may also be defined by the method of bonding in which light pressure can cause a bond to form.
Widely used pressure sensitive adhesives are hot melt pressure sensitive adhesives, which are commonly based on block copolymers. These adhesives are a 100% solid system and are applied to a substrate in the molten state and then cooled to ambient temperatures. They are often used for labels, including both removable, semi-permanent and permanent labels, as well as for removable articles such as feminine napkins, diaper tapes and so forth.
Water-based pressure sensitive adhesives are also commonly utilized, and are typically based on latex or styrene-butadiene emulsions. Both water-based and hot melt PSA's may be based on acrylate copolymers as well.
Solvent based pressure sensitive adhesives are also utilized and often include an acrylic-based polymer system. These types of adhesives are popular for use in medical applications, for instance, in transdermal drug delivery applications whereby the adhesive is used to adhere the article to the skin. The disadvantages to using solvent based adhesives are obvious and include the detrimental effects of solvent on health as well as flammability issues of using solvents in commercial plants.
A growing industry in which pressure sensitive adhesives are being utilized is in the medical field for applications such as medical tapes, wound dressings and bandages. In these applications, adhesives that are known in the art as "skin attachment adhesives" are used. These adhesives require low 180.degree. peel values (i.e., removability), low storage moduli, low glass transition temperatures and high moisture vapor transmission rates. As mentioned above, acrylic-based polymer systems are still widely used. Polyurethane-based pressure sensitive adhesives, in contrast, are not widely used because of the difficulty in achieving the viscoelastic characteristics necessary for a good pressure sensitive adhesive. U.S. Pat. No. 5,714,543 to Shah et al. issued Feb. 3, 1998 discloses a pressure sensitive adhesive that includes a polyurethane polymer that includes the reaction of a polyisocyanate with a polyol, and a dissolved water soluble polymer free from moieties that will react with isocyanate groups. An excess of polyol (i.e., hydroxyl functionality) relative to isocyanate is utilized resulting in a composition that has no isocyanate termination and the curing mechanism does not include moisture. The NCO:OH mole weight ratio is from about 0.5:1.0 to about 0.99:1.0, so that these polyurethanes have excess hydroxyl functionality. The composition is designed to have improved water uptake. The water soluble polymer additives enhance the water uptake.
U.S. Pat. No. 5,183,664 describes a thin film wound dressing that includes a backing layer having over one surface thereof a continuous adhesive layer. The adhesive includes a gel adhesive that is not self-adherent and preferably includes hydrophilic gel containing polyurethane residues. These polyurethane gels are based on polyoxyalkylene diol mono alkyl ethers. Polyoxyethylene-polyoxypropylene mono butyl ether, which is terminated with a hydroxyl group and with a butyl group, is disclosed as the preferred polyoxyalkylene diol mono alkyl ether. The polyurethane is cured by radiation but may be cured with a molar excess of water or with a stoichiometric amount of a polyol or polyol mixture.
U.S. Pat. No. 4,661,542 issued Apr. 28, 1987 to Gilch et al. discloses pressure-sensitive adhesives that are curable by atmospheric moisture and that include a prepolymer component with isocyanate functionality between 2.0 and 3.0 together with one or more polyetherurethane prepolymers having NCO groups available for reaction with water for chain extension. The cured adhesive composition has a glass transition temperature below 20.degree. C. The composition of Gilch et al. includes at least two polyurethane prepolymers, one of which has an isocyanate functionality between about 2.0 and 3.0 and one of which is a polyetherurethane prepolymer. The polyurethane prepolymer has an isocyanate functionality between about 2.0 and 3.0 may be a blend of polyetherurethane prepolymers of various functionalities. Gilch et al. disclose, "If desired, prepolymers of lower functionalities may be used in conjunction with low molecular weight polyisocyanates of functionality in excess of 2.0 in order to achieve a suitable NCO functionality of the prepolymer component" (see column 4, lines 54 to 64). "For example compositions according to the invention may be produced using branched chain polyethers having an average hydroxyl functionality in excess of 2.0, for example in the range of 2.4 to 2.8 or greater" (see column 5, lines 21 to 25).